A cyclotron is a type of circular particle accelerator in which negatively or positively charged particles are accelerated outwards from the centre of the cyclotron along a spiral path up to energies of several MeV. Unless otherwise indicated, the term “cyclotron” is used in the following to refer to isochronous cyclotrons. Cyclotrons are used in various fields, for example in nuclear physics, in medical treatment such as proton-therapy, or in radio-pharmacy. In particular, cyclotrons can be used for producing short-lived positron-emitting isotopes suitable for PET imaging (positron emitting tomography) or for producing gamma-emitting isotopes, for example, Tc99m, for SPECT imaging (single photon emission computed tomography).
A cyclotron generally comprises several elements including an injection system, a radiofrequency (RF) accelerating system for accelerating the charged particles, a magnetic system for guiding the accelerated particles along a precise path, an extraction system for collecting the thus accelerated particles, and a vacuum system for creating and maintaining a vacuum in the cyclotron.
A particle beam constituted of charged ions is introduced into a gap at or near the center of the cyclotron by the injection system with a relatively low initial velocity. As illustrated in FIG. 3, this particle beam is sequentially and repetitively accelerated by the RF accelerating system and guided outwards along a spiral path comprised within the gap by the magnetic field generated by the magnetic system. When the particle beam reaches its target energy, it can be extracted from the cyclotron by the extraction system provided at a point of extraction, PE. This extraction system can comprise, for example, a stripper consisting of a thin sheet of graphite. For example, H− ions passing through the stripper lose two electrons and become positive. Consequently, the curvature of their path in the magnetic field changes its sign, and the particle beam is thus led out of the cyclotron towards a target. Other extracting systems exist which are well known to the persons skilled in the art.
The magnetic system generates a magnetic field that guides and focuses the beam of charged particles along the spiral path until it is accelerated to its target energy. In the following, the terms “particles”, “charged particles”, and “ions” are used indifferently as synonyms. The magnetic field is generated in the gap defined between two magnet poles by two solenoid coils, 14, wound around these poles. Magnet poles of cyclotrons are often divided into alternating hill sectors and valley sectors distributed around a central axis. The gap between two magnet poles is smaller at the hill sectors and the larger at the valley sectors. A strong magnetic field is thus created in the hill gap portions within the hill sectors and a weaker magnetic field is created in the valley gap portions within the valley sectors. Such azimuthal magnetic field variations provide radial and vertical focusing of the particle beam every time the particle beam reaches a hill gap portion. For this reason, such cyclotrons are sometimes referred to as sector-focusing cyclotrons. In some embodiments, a hill sector has a geometry of a circular sector similar to a slice of cake with a first and second lateral surfaces extending substantially radially towards the central axis, a generally curved peripheral surface, a central surface adjacent to the central axis, and an upper surface defining one side of a hill gap portion. The upper surface is delimited by a first and second lateral edges, a peripheral edge, and a central edge.
It is difficult to manufacture a pair of magnet poles yielding a perfectly predictable magnetic field due, inter alia, to defects and or inhomogeneities in the steel used for the magnet poles, machining precision, as well as to differences between different batches of steel. For this reason, one lateral edge of a hill sector is often cut off to accommodate a lateral pole insert. Upon the results of calibration tests, said lateral pole insert is removed, machined to modify the topography of the upper surface and/or of the lateral surface thereof, and repositioned onto the hill sector. This operation allows the correction of the actual magnetic field and is repeated until it matches the target magnetic field. These iterative corrections including the removal, machining, and repositioning of a lateral pole insert can be long and cumbersome. This is particularly true because the same operations must be carried out identically on the lateral pole inserts of all the hill sectors.
There therefore remains a need in the art to provide an isochronous sector-focused cyclotron allowing an easy and cost effective fine tuning of the magnetic field formed at the hill gap portions between hill sectors to match the target properties thereof.